Arachidonic acid (20:4ω−6) and docosahexaenoic acid (22:6ω−3) are major acyl components of cell membrane phospholipids, and are particularly enriched in the nonmyelin membranes of the central nervous system. Dietary deficiency of linoleic acid (18:2ω−6) and linolenic acid (18:3ω−3) during development has been shown to result in reduced levels of 20:4ω−6 and 22:6ω−3 in the developing central nervous system, and this has been associated with altered learning behaviour and visual function. Synthesis of 20:4ω−6 and 22:6ω−3 depends on the dietary intake of 18:2ω−6 and 18:3ω−3, respectively, and the activity of the fatty acid desaturase–elongase enzymes. Oxidation of 18:2ω−6 and 18:3ω−3 for energy, or direct acylation of 18:2ω−6 into triglycerides, cholesteryl esters, and phospholipids, could also influence the amount of 20:4ω−6 and 22:6ω−3 formed. The tissue levels of 20:4ω−6 and 22:6ω−3, or other (ω − 6) and (ω − 3) fatty acids, compatible with optimum growth and development or health are not known. The amount of preformed 22:6ω−3 in the diet of adults, infants fed various milks or formulae, or animals is reflected in the circulating lipid levels of 22:6ω−3. Human milk levels of (ω − 6) and (ω − 3) fatty acids vary, depending in part on the mother's diet. A valid, scientific approach to extrapolate dietary essential fatty acid requirements from the composition of human milk or the circulating lipids of infants fed different diets has not been agreed on. Current data suggest that fatty acid requirements for development of term-gestation piglet brain and retina are met with 5.0% dietary kcal (1 cal = 4.1868 J) 18:2ω−6 and > 1.0% kcal 18:3ω−3, As in rodents and non-human primates, a diet source of 20:4ω−6 and 22:6ω−3 does not seem essential for the developing piglet central nervous system. However, studies in very premature infants suggest these infants may benefit from a dietary source of 20:4ω−6 and 22:6ω−3. Whether the low 20:4ω−6 and 22:6ω−3 status is due to oxidation of 18:2ω−6 and 18:3ω−3 for energy, the effects of early intravenous feeding with lipid emulsions, rapid growth, or immaturity of physiological or metabolic pathways in very preterm infants is not yet known.Key words: linoleic acid, linolenic acid, arachidonic acid, docosahexaenoic acid, brain, retina.